When sponges take over

Overfishing, pollution, and most of all, climate change, are destroying corals, causing the collapse of ecosystems and fishing industries around the world.

“Corals are the backbone of the entire ecosystem,” said Emily Darling, a marine and climate change researcher at the University of North Carolina at Chapel Hill.

When corals die, sponges often take over the reefs, which causes drastic changes in reef ecosystem dynamics.

Think of an ecosystem like a city. Some cities are lively and diverse, have great restaurants and an eclectic music scene. Reefs with corals at their base are like this. Corals are colorful and vibrant, and most importantly, support a diverse group of plants and animals, including fish people eat.

Some cities are dull. Life in these cities is monotonous, there is little diversity, and there is absolutely nothing to do on a Friday night. These are reefs without corals. When corals die, other organisms such as sponges take over. Corals are “really critical for biodiversity,” said Elizabeth McLeod, climate adaptation scientist for The Nature Conservancy.

Global warming is making the oceans hotter and saltier, which destroys the tiny algae that live inside corals and give corals their bright colors. Without these algae, corals die. This process is called coral bleaching because killing the algae removes all color from the corals, leaving them white and “bleached.” Coral bleaching is “one of the most critical climate change impacts,” McLeod said.

Human actions that release carbon dioxide into the air, such as burning fossil fuels, also release carbon dioxide into the oceans. More carbon dioxide lowers the water’s pH, making it more acidic, a stressful state for most corals. More carbon dioxide in the water also means less carbonate, which corals need to build their skeletons.

Overfishing is another important threat to corals, McLeod said. Overfishing removes fish at the top of the food chains in coral reefs. Without these fish to eat organisms further down the chain, the food chain breaks down, and the reef ecosystem, including the corals, collapses.

When corals die, they leave behind open space for other organisms such as sponges, urchins and certain species of algae. Most marine researchers once thought algae to be the most likely contender for taking over when corals die. New research shows sponges may have a better chance than previously thought.

In a paper published in the May 2013 issue of Global Change Biology, James Bell, marine ecologist at Victoria University of Wellington, cited cases in Belize, Puerto Rico, Indonesia and at Palmyra Atoll in the Central Pacific Ocean where the area covered by coral had decreased while the number of sponges increased.

One sponge whose takeover has been better documented than most is a brown, bulbous sponge called the chicken liver sponge. In 1998, oceans reached record temperatures, destroying a record number of corals through coral bleaching. Some types of corals experienced greater than 90-percent mortality. Scientists refer to this as the mass bleaching event of 1998.

Richard Aronson, professor and head of biological sciences at the Florida Institute of Technology, conducted research in Belize around this time and found the chicken liver sponge almost completely covered the sea floor which had once been dominated by corals. The chicken liver sponge’s success was due to how fast it grows (many times the rate of most corals), and the smelly toxins it produces to ward off predators.

Researchers have documented similar takeovers by sponges across the Caribbean and Pacific Ocean where corals have died as oceans got hotter, more acidic and more polluted. Sponges have survived these changes mainly because they are less sensitive than corals.

When sponges take over, the ecosystem becomes a dull city. “It’s not like it’s a wasteland,” said John Bruno, marine ecologist at the University of North Carolina at Chapel Hill, “but I doubt that [sponges] are going to support fish populations in the way that corals will.” This means lower diversity overall as well as fewer edible fish.

“It’s hard to see any real benefits either ecologically or economically” when sponges take over, said Bell, author of the journal article on sponge dominance.